Embodiments of the present invention relate to power converters and more specifically to a multilevel converter.
In the last few decades, the field of power conversion has grown tremendously due to its imminent advantages in motor drives, renewable energy systems, high voltage direct current (HVDC) systems, and the like. A power converter which converts electrical energy from one form (e.g., alternating current (AC) or direct current (DC)) to another form (e.g., DC or AC) utilizes various power electronic devices. Examples of power electronic devices are insulated gate bipolar transistors (IGBTs) and thyristors.
Thyristors have very low conduction losses compared with other power electronic devices such as IGBT. In addition, thyristors can guarantee to fail short, so for the applications which need to connect power electronic devices in series to block high voltage, a system can continue operating even if one or more thyristors fail. This feature of thyristors enables a true redundant design, leading to much higher system reliability. These two features make the thyristor an ideal device for the HVDC application which requires high voltage, high efficiency and high reliability. However, thyristors are only partially controllable devices and thus cannot be turned off as fully controllable devices such as IGBT. Therefore, thyristors are generally used in a load commutated converter (LCC) based classic HVDC systems, in which thyristors are turned off with the help of an AC grid.
Due to the switching limitations of thyristors, IGBTs are usually considered as the main workhorse for a voltage source converter (VSC) based HVDC system. The VSC based HVDC system compared with the LCC based classic HVDC has many potential benefits: 1) can support passive network, or act as the grid for an island; 2) can control reactive power independent from active power. However, IGBTs lack the above mentioned features of thyristors.
Therefore, there still exists a need for an economically more viable as well as technically reliable and efficient alternative solutions for voltage source converters.